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Although key determinative events of the Rhizobium-legume symbiosis are likely to precede bacterial infection, no plant genes have been identified that are expressed strongly prior to infection and nodule morphogenesis. A subtractive hybridization-polymerase chain reaction technique was used to enrich for genes induced during the early phases of the R. meliloti-Medicago truncatula symbiosis. One gene so identified encodes a putative plant peroxidase protein, which we have named Rip1 for Rhizobium-induced peroxidase. The accumulation of rip1 transcript was rapidly and transiently induced by R. meliloti and by the corresponding lipooligosaccharide signal molecule Nod factor RmIV, which was both necessary and sufficient for rip1 induction. The duration of maximal rip1 expression coincided with the preinfection period: transcript levels for rip1 were near maximal by 3 hr postinoculation and declined by 48 hr, coincident with early infection events and the onset of nodule morphogenesis. Furthermore, although rip1 induction preceded bacterial infection by at least 24 hr, the transcript was localized to epidermal cells in the differentiating root zone that was subsequently infected by Rhizobium. Thus, a defining feature of the Rhizobium infection court is the prior induction of rip1 expression.

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